Continuous as well as discontinuous methods are known for the production of paste-grade polyvinyl chloride. The vinyl chloride polymer produced by the continuous process results in plastisols having low viscosities at high shear rates. However, greater amounts of emulsifier are needed for their production, leading to reduced transparency and higher water sensitivity. The vinyl chloride polymers normally obtained by continuous polymerization below the vinyl chloride saturation pressure are also inferior to the discontinuously polymerized polyvinyl chloride in regard to thermostability.
The types of polyvinyl chloride produced by the dicontinuous method can be polymerized by emulsion polymerization with markedly lower quantities of emulsifier, especially if the emulsifier is added in accordance with the method of German Patent 1,964,029 or according to the laid-open disclosure of Belgian Patent 656,985. However, in all cases, plastisols are obtained having viscosities that are higher than in continuous polymerization.
Also conventional is vinyl chloride polymerization according to the so-called microsuspension process. Plastisols from vinyl chloride polymers prepared by this process exhibit a pronouncedly dilatant behavior and thus are less suitable for being processed by the spread coating method. Besides, stable latices can be prepared only up to a solids content of about 40% by weight, which increases the spray-drying costs.
Various discontinuous methods have been known for vinyl chloride polymerization with the use of dispersing aids, in most cases fatty alcohols. The following can be cited as state of the art:
1. DOS 2,850,105 PA1 2. DOS 2,742,178 (U.S. Pat. No. 4,093,581) PA1 3. European Patent 0,030,524 PA1 4. German Application P 32 10 891.5 (U.S. application Ser. No. 478,766 of Mar. 25, 1983) PA1 5. German Application P 32 42 088.9 (U.S. application No. 551,033 of Nov. 14, 1983) PA1 (a) an alkali metal salt or ammonium salt of a branched or unbranched fatty acid containing 12-18 carbon atoms, of a branched or unbranched alkylsulfonic acid containing 10-20 carbon atoms, of an alkylarylsulfonic acid containing 8-18 carbon atoms in the branched or unbranched alkyl chain, or of a sulfosuccinic acid ester containing 6-14 carbon atoms in the alcohol portion, in amounts of 0.2-3.0% by weight, based on the monomer, PA1 (b) a straight-chain or branched C.sub.12 -C.sub.20 -alkanol in amounts of 50-200% by weight, based on the tenside employed, PA1 (c) water, as well as optionally PA1 (d) a monomer-soluble catalyst, wherein the predispersion is prepared only with 30-80% by weight of the amount of emulsifier (a) (amount of tenside) required in total, and is added to the polymerization batch, whereas the remainder of the emulsifier, or of a component forming the emulsifier, is added in metered quantities batchwise or continuously as an aqueous solution to the polymerization mixture after a conversion of 10-60% of weight. Preferably, only 40-60% by weight of the amount of emulsifier required in total is utilized for preparing the predispersion. In particular, the remainder off the emulsifier can be added after a conversion of 20-60% by weight. PA1 (a) alkali metal salts or ammonium salts of fatty acids, of alkylsulfonic acids or alkylarylsulfonic acids or sulfosuccinic acid esters in quantities of 0.2-3% by weight, preferably 0.5-1.5% by weight, based on the monomer, and PA1 (b) a straight-chain or also branched C.sub.12 -C.sub.20 -alkyl alcohol or mixture of several such alcohols in amounts of 50-200% by weight, preferably 70-150% by weight, based on the tenside (a) employed.
Polymerization according to method 1 is conducted by means of oil-soluble activators, partially with the addition of reducing agents. This process leads to products which, when processed into plastisols, exhibit a strongly dilatant flow characteristic, as can be seen from Comparative Experiment A of German Patent Application P 32 10 891.5.
The process described in (2) utilizes inorganic catalysts for the activation. Accordingly, a mixture is employed during polymerization consisting of a C.sub.16 -C.sub.20 -alkyl alcohol and an alkyl sulfate (C.sub.12 -C.sub.18). As demonstrated by Comparative Experiments B and C of German Patent Application P 32 10 891.5, latices are obtained according to (2) which are either unstable or of a low solids content, yielding a polyvinyl chloride upon spray-drying which results in high-viscosity pastes with a pronounced pseudo-plastic flow behavior.
According to (3), a water-soluble material is first homogenized in the presence of water and emulsifier. Thereafter, the monomer and initiator are added thereto. In order to conduct the process successfully, the presence of a seed latex (stabilizing latex) is required, and thus the process is comparatively expensive.
According to (4), though low-viscosity pastes are obtained, increased technical expenditure is required. This is so because the predispersion to be added during polymerization must be prepared in a separate, heatable agitated vessel and must be heated throughout the entire polymerization procedure to above the melting point of the fatty alcohol employed. In spite of the heated action, the predispersions may change during the period of metered feeding outside of the vessel, leading to fluctuating product properties. Additionally, when processing the products prepared according to (4) into expandable plastisols with the use of chemical blowing agents, such as, for example, azodicarbonamide, foam materials are obtained after gelling which have a primarily closed-cell foam structure. This, as is known, results in foams having a lack of elastic memory capacity.
The process listed in (5) likewise requires increased technical expenditure, since a pressure homogenizer is utilized.